Field
The present invention relates to vibrating microelectromechanical (MEMS) gyroscopes and to continuous monitoring of their operation.
Brief Description of the Related Art
MEMS gyroscopes use the Coriolis Effect to measure angular velocity. In a vibrating MEMS gyroscope a mass element is driven into oscillating movement by an actuating drive force. This oscillation will be called “drive oscillation” in this disclosure and it can be either linear or rotational. FIG. 1 illustrates a mass driven in linear oscillation along its primary axis a1, while FIG. 2 illustrates a mass driven in rotational oscillation about its primary axis a2. The drive oscillation is indicated with a solid black arrow in both figures. The actuating drive force can be generated, for example, with an electrostatic, magnetic or piezoelectric actuator.
A microelectromechanical gyroscope may comprise a reference frame to which one or more mass elements are connected with spring structures which allow the mass element to move with at least two degrees of freedom. The reference frame typically surrounds the mass elements. The reference frame and the mass elements are typically configured for electrical measurements in relation to each other. The mass element can oscillate within the plane defined by the reference frame or out of the plane defined by the reference frame.
When the gyroscope containing a mass element in drive oscillation undergoes an angular rotation rate Ω about a secondary axis which is not parallel to the primary axis, the mass element is affected by the Coriolis force. The Coriolis force is determined by the magnitude and direction of the angular rotation rate vector and the mass element velocity vector. A mass element in drive oscillation will undergo an oscillating Coriolis force. This force oscillates the mass element along or about a secondary axis perpendicular to the primary axis. Oscillation along or about the secondary axis will be called “sense oscillation” in this disclosure.
In FIGS. 1 and 2 the angular rotation rate Ω is indicated with a white arrow and the sense oscillation is indicated with a grey arrow. The sense oscillation is a linear oscillation along axis a3 in FIG. 1 and a rotational oscillation about axis a3 in FIG. 2. In other words, the secondary axis is a3 in both FIGS. 1 and 2. The sense oscillation may be detected through capacitive, piezoelectric or piezoresistive sensing in relation to a fixed reference frame. The electric signal arising from this detection will be called the “sense signal” in this disclosure.
In this disclosure the term “sense oscillation amplitude” means the maximum extent that a mass element in the gyroscope is displaced from its rest position as it undergoes sense oscillation. The term “drive oscillation amplitude” means the maximum extent that a mass element in the gyroscope is displaced from its rest position as it undergoes drive oscillation. Both the sense oscillation amplitude and the drive oscillation amplitude can be either linear distances or angles, as is evident from FIGS. 1 and 2.
Correspondingly, in this disclosure the terms “sense oscillation frequency” and “drive oscillation frequency” mean the frequency at which a mass element oscillates in sense oscillation and drive oscillation, respectively. The symbol ωD represents the drive oscillation frequency in this disclosure. The frame, the mass element and the springs may be designed so that the sense oscillation frequency assumes a value which is equal or very close to the drive oscillation frequency.
Several mass elements can be driven with one drive actuator. The drive motion is transmitted simultaneously to all mass elements. Simultaneous capacitive monitoring of several mass elements facilitates measurement of various differential capacitances. Differential capacitance measurements are less noisy than single-sided measurements because they allow the sense oscillation (which arises from the angular rate) to be more clearly separated from additional capacitive signals (which can arise from other vibrations or accelerations). One way to achieve a clear separation is to drive the mass elements in opposite phases (by, for example, driving one mass in the positive direction along the a1-axis when another moves in the negative direction, and vice versa). The sense oscillation of these mass elements will then also be in opposite phase (one mass will move in the positive direction along the a3-axis when the other moves in the negative direction, and vice versa). This differential-mode sense oscillation (arising from the angular rate) can be clearly distinguished from common-mode motion (arising from other vibrations or accelerations) where both masses move in the same direction along the a3-axis.
An additional benefit of several mass elements is that an oscillating internal angular momentum (arising from the drive motion) about the center of the gyroscope is avoided when several mass elements are positioned symmetrically around the center and set to oscillate in a suitable phase.
FIG. 3 illustrates a MEMS gyroscope where centrally located drive actuator 33 actuates a linear drive oscillation in two adjacent mass elements 31 and 32. FIG. 4 illustrates a MEMS gyroscope where the centrally located drive actuator 43 actuates a rotational drive oscillation in two adjacent mass elements 41 and 42. In both gyroscopes the two mass elements are typically driven to oscillate in anti-phase, so that when 31 moves left, 32 moves right, and when 41 rotates clockwise, 42 rotates counter-clockwise.
The drive actuator may be, for example, a capacitive comb drive where an oscillating electrostatic force is generated between adjacent comb electrodes by connecting them to an oscillating drive voltage. This oscillating voltage will be called the drive signal in this disclosure. The drive signal has a drive signal amplitude and a drive signal frequency. The drive oscillation frequency ωD follows the drive signal frequency because the drive signal frequency is transmitted by the actuator directly to the mass element. Correspondingly, the drive signal amplitude determines the drive oscillation amplitude. In other words, the value of the drive oscillation frequency can be changed by adjusting the drive signal frequency and the value of the drive oscillation amplitude can be changed by adjusting the drive signal amplitude. The drive signal frequency is usually set to a value which is equal or very close to the resonance frequency of the mass element in drive motion to maximize the oscillation amplitude by the resonance gain.
While the drive oscillation is set by the drive signal in the manner described above, the sense oscillation is a more complicated movement. One component of the oscillation is determined by the drive oscillation amplitude and by the strength of the Coriolis force. The sense oscillation may have other components as well, as will be described in the detailed description of this disclosure. One objective in gyroscope design may be to keep the drive oscillation amplitude as constant as possible. An unintended and undetected change in drive oscillation amplitude would change the sensitivity of the gyroscope and produce erroneous measurement results.
There are many reasons why the drive oscillation amplitude may not remain exactly constant for long periods of time. Temperature is one important factor because thermal expansion may alter the mechanics of drive oscillation. A temperature change also influences the dynamic pressure exerted by the surrounding gas on the moving mass elements. The drive signal voltage may therefore have to be increased or decreased to maintain the same drive oscillation amplitude as the temperature changes.
The drive oscillation amplitude may also be deliberately changed based on temperature input, to keep the sensitivity of the gyroscope constant as the temperature changes. The extent of this temperature correction has to be determined beforehand in a temperature calibration procedure. This drive oscillation amplitude adjustment can be used either as a complement or as an alternative to numerical temperature corrections, where the measurement result is scaled with temperature correction factors.
Drive oscillation mechanics may also change due to various unexpected faults during operation. Production defects or mechanical wear in the active parts of the gyroscope or the bonds between them can affect its operation either gradually and suddenly. Vibrating microelectromechanical gyroscopes are therefore commonly equipped with a measurement arrangement for monitoring the drive oscillation amplitude. Such an arrangement is also required for the temperature calibration procedure mentioned above.
By continuously measuring its own drive oscillation amplitude and frequency, a vibrating microelectromechanical gyroscope can be programmed to automatically adjust the drive signal, either to compensate for an unexpected amplitude change or to perform a temperature correction. The gyroscope can also report its own malfunction to an external circuit in case disturbances in drive oscillation amplitude are too severe to be compensated.
Document U.S. Pat. No. 8,820,136 B2 discloses a microelectromechanical gyroscope with self-test capability. A mass element is driven in linear oscillation along the drive axis X. The mass element is also movable in the direction of an orthogonal sense axis Y. A self-test functionality is implemented with dedicated self-test electrodes on the mass element and the fixed frame. An oscillating drive signal is converted into an oscillating self-test signal with a lower self-test frequency than the drive oscillation. This self-test signal produces an oscillating force which acts on the mass element in the Y-direction. The signal measured from the movement of the mass element in the Y-direction is then continuously demodulated at the drive oscillation frequency to measure the influence of the Coriolis Effect. It is also demodulated at the self-test frequency to monitor if the gyroscope is still working properly. The problem with this prior art solution is that it requires separate self-test excitation electronics and a separate self-test excitation actuator. A gyroscope with this self-test functionality therefore requires more chip area and consumes more current than a gyroscope without self-test functionality. The additional excitation electronics and actuator also introduce additional complexity and a potential source of measurement error.
Document US20150211853 A1 also discloses a microelectromechanical gyroscope with self-test capability. In this document a linear drive actuator creates a rotational oscillation in the sense mass. The self-test functionality is implemented by including separate sensing electrodes for continuously monitoring the drive oscillation. The problem with this solution is that additional electrodes and circuitry have to be introduced for monitoring the drive oscillation. Furthermore, there is a risk of crosstalk between the drive signal and the drive monitoring signal, which increases the risk of measurement errors.